Extrusion-coated, biaxially oriented polypropylene film having improved adhesion characteristics

ABSTRACT

This invention relates to a biaxially oriented film of isotactic polypropylene which has improved adhesion characteristics, the film being provided on at least one surface with a coating of an isotactic polypropylene or a mixture of isotactic polypropylene and up to 15 percent by weight, based on the total weight of the polymer, of nonisotactic polypropylene, or a mixed polymer or copolymer of propylene and up to 15 percent by weight, based on the total weight of the polymer, of ethylene, the coating being substantially unoriented in the first stretching direction and less oriented than the base film in the second stretching direction.

United States Patent 11 1 3,620,825

[72] Inventors Harald Lolunann [56] References Cited WiesbIden-Bkbrkh:UNITED STATES PATENTS 2222"" 2,927,047 3/1960 Schulde et al. 1 17/1ss.sE [211 p No 809 3,258,340 6/1966 Riboni l17/138.8E

[22] Filed M". 21, 1969 3,285,766 11/1966 Barkrs et al. 117/138.8 E 45Patented Nov. 16, 1971 FORElGN PATENTS 73 Assign g u ut mn 700,48212/1964 Canada 1 17/138.8 E

wmnkbflch my Primary Examiner-William D. Martin Pnonty 1968 AssistantExaminer-Ralph l-lusack At 028.7 torney James E. Bryan [54] ABSTRACT:This invention relates to a biaxially oriented film ADHESIONCHARACTERISTICS of isotactic polypropylene which has improved adhesion 2cum 2 Drum. characteristics, the film being provided on at least onesurface m with a coating of an isotactic polypropylene or a mixture ofUS- I l7/l38.8 E, isotactic polypropylene and up to 15 percent byweight, based 117/7, 117/68, 117/76 F, "7/122 l-l, 117/161 1 on thetotal weight of the polymer, of nonisotactic H, l 17/ 161 UPpolypropylene, or a mixed polymer or copolymer of propylene 1532b 27/32,and up to 15 percent by weight, based on the total weight of B32h 27/08the polymer, of ethylene, the coating being substantially [50] FieldotSarcl: 1 117/ 138.8 unoriented in the first stretching direction andless oriented E, 161 UF, 161 H; 161/252; 1 17/7 than the base film inthe second stretching direction.

PATENTEmuv 16 l9?! 3. 620. 825

FIG. 2

INVENTORS HARALD LOHMANN HORST DlENER BY U d} ATTORNEY EXTRUSION-COATED,BIAXIALLY ORIENTED POLYPROPYLENE FILM HAVING IMPROVED ADIIESIONCHARACTERISTICS This invention relates to a biaxially oriented film ofisotactic polypropylene which has extrusion-coated thereon, on one orboth surfaces, a relatively thin adhesive layer of isotacticpolypropylene or of mixtures containing isotactic polypropylene.

Films of isotactic polypropylene which have been oriented in onedirection or in two orthogonal directions by stretching them at atemperature below their crystallite melting point are suitable for manyfields of application, because of their mechanical strength, theiroptical clarity and surface gloss, and their low vapor permeability.

However, such films have the significant drawback that, due to thenonpolar character of the material and its high degree of molecularorientation, their adhesion e.g. to heat-sealing layers, metal layersand the like is poor.

Polyethylene or the so-called "hot melts" and copolymers based on vinylacetate, vinyl chloride or nitrocellulose are normally used asheat-sealing layers. If it is desired to additionally provide thecomposite film with good barrier properties towards gases, such as CO,or O,properties which are increasingly being required of packagingfilms-the oriented polypropylene films are provided with heat-sealinglayers based on vinylidene chloride copolymers. For metallizing films,e.g. by vapor deposition, aluminum is normally used, but copper, silveror gold also may be employed for special purposes.

For making the surface of biaxially stretched isotactic polypropylenefilms more suitable for a subsequent coating or metallization process,numerous methods have been proposed, such as a treatment of the filmswith electric discharges in an atmosphere of air or of special gases,treatment with chemicals, or flame treatment. Surfaces pretreated inthis manner form satisfactory composite films when they are combinedwith copolymers based on vinylidene chloride, vinyl chloride, or vinylacetate, provided these substances are applied from suitable organicsolvents.

The application of coatings from organic solutions has, however,essential disadvantages from the point of view of processing; itrequires, e.g., precautionary measures to avoid the danger of explosionand makes it necessary to remove or recover the solvent used. Further,the heat-sealing layers produced in this manner poses no antistaticproperties and may be physiologically objectionable. in the case of theimportant heat-sealing layer based on vinylidene chloride, the barrierproperties are, moreover, not fully satisfactory.

For these reasons, there has been a tendency to apply the heat-sealinglayers from aqueous dispersions. However, in this case the adhesion ofthe coating to the supporting film is insufficient for most purposes,despite an intensive pretreatment. Hitherto, this obstacle has been met,on the one hand, by modifying the supporting film in such a manner, bythe addition of polyethylene, copolymers of propylene and ethylene, ormixtures of polyethylene and polypropylene, that the subsequent coatingprocess yielded composite films of improved bonding strength. On theother hand, for further improving the adhesion, the already pretreatedsurface was coated from an aqueous dispersion or an organic solutionwith an additional base coat or an anchoring layer, and the actualheatsealing layer having the desired properties was then applied as atop layer, in a second operation.

The application of an additional adhesive layer from an organic solutioncauses the above-mentioned processing drawbacks, whereas the applicationfrom an aqueous dispersion results in a bonding strength which is notsufficient for all purposes.

Moreover, the modification of the homopolymeric isotactic polypropyleneby forming copolymers thereof or by adding From what is said above, thecompromises can be seen which have hitherto been necessary due to thepoor adhesion of layers to the surface of biaxially stretchedpolypropylene.

The present invention provides a biaxially oriented film of isotacticpolypropylene, the surface of which is modified in such a manner thatthe adhesion of subsequently applied coatings is improved.

In the present invention, a biaxially oriented film of isotacticpolypropylene is provided on at least one surface with a layer of anisotactic polypropylene, or a mixture of isotactic polypropylene and upto 15 percent by weight, based on the total weight of the polymer, ofnonisotactic polypropylene, or a mixed polymer or copolymer of propyleneand up to l5 percent by weight, based on the total weight of thepolymer, of ethylene, which layer is substantially unoriented in thefirst stretching direction and less oriented than the base film in thesecond stretching direction.

isotactic polypropylenes having a density of 0.880 to 0.920 and meltindices i at 230 C. from 4 to I00 g./l0 min., preferably from 10 to 50g. per 10 minutes, measured in accordance with method D 1238-57 T, aresuitable for the purposes of the present invention.

Mixtures of isotactic polypropylene containing up to 15 percent byweight, based on the total weight of the polymer, of nonisotacticpolypropylene, or mixed polymers or copolymers of propylene and up to 15percent by weight, based on the total weight of the polymer, ofethylene, may be used as coating materials. Preferably, an isotacticpolypropylene is used the fluidity of which may differ from that of thebase film. The use of one and the same isotactic polypropylene for theproduction of the base film and of the coating is particularlyadvantageous.

The composite film consists of a supporting or base film of biaxiallystretched isotactic polypropylene of any desired thickness, which isprovided on at least one surface with a coating of the above-describedpolymers, mixed polymers or copolymers, of a thickness of 0.2 to 4 pm,preferably 0.5 to 1.5 am.

The composite film according to the invention is prepared by extruding alayer of l to 20 um. thickness, preferably 3 to 10 um. thickness-ifdesired after a previous treatment to improve adhesion-onto one or bothsurfaces of a monoaxially stretched base film obtained after the firststretching operation-which in this case is preferably in thelongitudinal direction-of a known process for the biaxial stretching offilms of isotactic polypropylene, e.g. by means of the so-calledstretching frame method, and then stretching the composite film thusproduced transversely to the direction of the first stretchingoperation. If necessary, the two surfaces may be coated with differentpolymers. As can be seen from the following examples, particularlyexample I, contrary to the first stretching operation, the secondstretching operation is performed at a temperature near the crystallitemelting point of the polypropylene. This stretching temperature. whichis higher than the temperature employed for the first stretchingoperation, is necessary, since it is known that a film material thatalready has been oriented in one direction-in this case the basefilmrequires considerably higher energies when it is to be oriented inthe orthogonal direction. However, for the coating applied, which hasnot yet been oriented in one direction, this temperature is so highthat, due to the high molecular mobility, a degree of orientation isachieved which is lower than that of the base film in the secondstretching direction. In the first stretching direction, the coatingfilm is only insignificantly oriented, as is normal with unstretchedpolypropylene films.

As the next step or, alternatively, after an intermediate storage, thesurface of the composite film produced in accordance with the inventionthen may be treated by one of the known methods and provided on one orboth surfaces with a layer. Thus, a layer of a heat-scalable material,e.g. a polyvinylidene chloride copolymer, may be applied from an aqueousdispersion, whereupon the composite film is dried and woundSurprisingly, the heat-scalable film thus produced displays aconsiderably improved adhesion of the heat-sealing layer to the filmprecoated in accordance with the invention. No additional anchoringlayer is necessary. Furthermore, the process can be conducted in asubstantially simpler manner. The extrusion of isotactic polypropyleneonto a monoaxially stretched base film of isotactic polypropylene can beeffected without difficulty. In addition thereto, the good opticalproperties of the base film are retained.

The advantages of the heat-sealable film produced in accordance with thepresent invention, as regards quality and processing conditions, will bedescribed in detail by reference to the following examples and thedrawings attached.

The evaluation of the qualities of the coated films as given in theexamples is based on the following measuring methods: a. The sealingstrength is determined using a strip 1 cm. wide which is cuttransversely to the sealing weld of the sample. The force in grams percm. of strip width measured during delamination in a tension tester at afeed speed of 200 mm./min., serves as a measurement of the sealingstrength.

The clamped free ends of the sample should form an angle of about 90with the still undelaminated film. The sealing weld, which has a widthof 15 mm., is produced in a pneumatic sealing device under constantconditions of a temperature of 120 C., a sealing time of 2 seconds, anda pressure of 1.3 kgJcmF.

b. Sealing strength after boiling for minutes: the test for determiningthe sealing strength described sub (a) was made after the sample hadbeen immersed for 5 minutes in boiling water.

c. For determining the bonding strength of a polypropylene/polyethylenecomposite film, the composite film is first cut from the polyethylenesurface in such a manner that the polypropylene layer is not damaged.The thus loosened laminate is opened over 15 mm. and the sample is thencut into mm. wide strips. The bonding strength is determined by theso-called T-peel test, at a feed speed of 100 mm./min. For this test,the clamped ends of the base film and of the coating film are drawnapart at an angle of 180, while the undelaminated composite film is heldat an angle of 90 to this direction of movement.

d. The adhesion of a metal layer is determined as follows: A 25 mm. widestrip of an adhesive tape which is available under the designationTesafilm" 104, is rolled onto the metal layer with a force of 2.5 kg.,using a roller of 40 mm. diameter. The laminate is opened over mm. andclamped into two clips connected to a device for measuring the force.The laminate is then drawn apart by the T-peel method at a feed speedwhich increases from 2.5 mm./sec. to 150 mm./sec. The force at which themetal layer begins to separate from the polypropylene film and adheresto the adhesive tape, is designated as the bonding strength.

EXAMPLE 1 isotactic polypropylene having a density of 0.91 and a meltindex i, at 230 C. of 15 g./l0 min., measured the ASTM method D 1238-57T, was extruded and longitudinally stretched at 145 C. at a rate ofA,=6.0. The thickness of the longitudinally stretched film 2 (see F 16.l) was 200 um.

Then, a hot melt at 280 C. of the above-described polypropylene wasapplied in two layers (layers 1 and 3 of FIG. 1) of a thickness of about6 pun. each, to both surfaces of the monoaxially stretched polypropylenefilm, at a rate of 60 m./min., and the composite film was then cooled to25 C. Thereafter, the film was transversely stretched at a stretchingrate A -8.0 and a stretching temperature of about 165 C. A biaxiallyoriented film 5 (FIG. 2) of a thickness of 25 pm. was thus obtainedwhich was coated on both surfaces with layers of oriented isotacticpolypropylene of about 0.8 gm. thickness (4 and 6 in FIG. 2) which wereonly insignificantly oriented in the longitudinal direction and lessoriented than the base film in the transverse direction.

The composite film was passed through an electric discharge and thencoated on both sides with an aqueous dispersion of a polyvinylidenechloride copolymer of percent of vinylidene chloride and 10 percent ofacrylonitrile and acrylic acid ester (thickness of the dried layer: 3p.m.),-dried at 120 C. in a hot-air drier, and wound up.

The heat-scalable film produced in this manner had the followingcharacteristics:

l-leat-seallng strength: l20-140 gJcm. Heat-sealing strength aficrboiling for 5 min.: 140 g./cm.

A comparison film which had been produced by the same method, but notcoated with isotactic polypropylene after longitudinal stretching, hadthe following properties:

Heat-sealing strength: 30-50 gJcm. Heat-sealing strength after boilingfor 5 min.: 30-50 gJcm.

' EXAMPLE 2 A heat-scalable film was prepared as described in example 1.Additionally, the longitudinally stretched polypropylene.

film was treated with an electric discharge before extrusion coating.The resulting film had the following characteristics:

Heal-sealing strength: Heat-sealing strength afier boiling for 5 min.:

-150 gJcm.

-140 g./cm.

EXAMPLE 3 A heat-sealable film was again prepared as described inexample 1, with the exception that in addition to the treatment with anelectric discharge, the longitudinally stretched film was cooled only to100 C., extrusion coated at this temperature, and then transverselystretched as in example 1. The following values were obtained:

Heat-sealing strength: 120-160 gJcm. Heat-sealing strength after boilingfor 5 min.: l30-l50 gJcm.

EXAMPLE 4 A longitudinally stretched film of isotactic polypropylenehaving the physical characteristics stated in example 1 wasextrusion-coated on one surface with a layer of about 6 pm. thickness ofthe same isotactic polypropylene, to which 10 percent by weight ofatactic polypropylene of a density of 0.86 had been added. After thesecond stretching operation, the film was provided as in example 1 witha heat-scalable layer for which the following characteristics weredetermined:

Heat-sealing strength: 120-140 Jeni. Heat-sealing strength after boilingfor 5 min.: 120-140 Jam.

EXAMPLE 5 ing characteristics were determined:

Heat-sealing strength: 120-160 g./cm. Heat-sealing strength afterboiling for 5 min.: l20-l 60 g./cm.

EXAMPLE 6 A longitudinally stretched film of isotactic polypropylenehaving the physical characteristics described in example 1 wasextrusion-coated on one surface with a layer of about pm. thickness of acopolymer consisting of 93 percent by weight of propylene and 7 percentby weight of ethylene and having a melt index i, at 230 C. of 13.1 g./l0min., measured by the ASTM method D 1238-57 T.

Afier the second stretching operation, the film was provided as inexample i with heat-sealable layer of which the followingcharacteristics were determined:

Heat-sealing strength: IOO-l 50 gJcm. Heat-sealing strength afterboiling for 5 min.: l00-l50 g./crn.

EXAMPLE 7 A film of isotactic polypropylene which had beenlongitudinally stretched as described in example 1 was extrusioncoated,by means of an appropriate two-component nozzle, on one surface with a 4pm. thick layer of isotactic polypropylene of the same characteristicsas in example 1 and with a 16 pm. thick layer of polyethylene of adensity of 0.918 in such a manner that the polypropylene layer wasbetween the longitudinally stretched polypropylene film and thepolyethylene layer. The composite film thus produced was transverselystretched at a temperature of 160 C. and a stretching rate A 8.0 andthen wound up.

In order to determine the bonding strength of the polyethylene coatingon the polypropylene layer more accurarely, instead of the usualevaluation of the heat-sealing strength, the composite film produced wasadditionally extrusion-coated on its polyethylene surface with a 75pmathick layer of polyethylene of a density of 0.918, so that thepolyethylene coating could be peeled off without breaking.

The bonding strength was 30-40 g./cm., whereas a composite film whichhad been prepared in the same manner, but without an intermediate layerof polypropylene, had a bonding strength of only 15-20 ycm.

EXAMPLE 8 A biaxially stretched polypropylene film which had seenprepared as described in example 1 was provided with an aluminum layerin the normal manner, by vacuum deposition.

The bonding strength of the metal layer to the po polypropylene film was400 g./cm., whereas a composite material which had been prepared in thesame manner, but without an intermediate layer of polypropylene, had abonding strength of only 240 g./cm.

It will be obvious to those skilled in the art that many modificationsmay be made within the scope of the present invention without departingfrom the spirit thereof, and the invention includes all suchmodifications.

What is claimed is:

1. A biaxially oriented film of isotactic polypropylene of improvedadhesion characteristics having a coating on at least one surfacethereof consisting essentially of an isotactic polypropylene; or amixture of isotactic polypropylene and not more than 15 percent byweight, based on the total weight of the polymer, of nonisotacticpolypropylene; or a mixture of polypropylene and not more than 15percent by weight, based on the total weight of the polymer, ofpolyethylene; or a copolymer of propylene and not more than 15 percentby weight, based on the total weight of the copolymer, of ethylene; thecoating being substantially unoriented in the first stretching directionand oriented but less oriented than the base film in the secondstretching direction, and the coating having a thickness in the range ofabout 0.2 to 4 pm.

2. A film according to claim 1 in which the coating is of the sameisotactic polypropylene as the base film.

i i i 0 i

2. A film according to claim 1 in which the coating is of the sameisotactic polypropylene as the base film.